This invention relates to a voltage surge arrestor, and more particularly to an arrestor having two operating voltage levels.
Surge arrestors are designed to shunt a voltage surge appearing on an electric lead to ground and thereby protect an electric component connected to the lead from damage. Arrestors of this nature are well known and are particularly useful with submersible motors for pumping ground water. As described in the Streater U.S. Pat. No. 3,849,704, such a motor is well grounded by the well water and it could be damaged by a voltage surge on a power lead due to lightning. Damage would normally require removal of the motor from the well and installation of another motor, which are expensive operations as well as an inconvenience to the user.
The above Streater patent shows a construction wherein surge arrestors are mounted within the interior of a motor housing or shell. This is an advantageous arrangement because the motor manufacturer may ensure that an arrestor of the proper design is provided. Further, an arrestor within a motor provides more effective protection than one connected to a motor by a long lead.
A difficulty arises, however, when an internal arrestor is provided within a sealed motor, and the motor must undergo high-voltage insulation tests before use. For example, motor standards require that the motor insulation be able to withstand a specified high voltage (such as 1,000 plus twice the rated RMS voltage) for a specified time (such as one minute). This voltage is between the motor frame and the stator windings and is designed to test the effectiveness of the motor insulation.
At the same time, the desired breakdown voltage for a surge arrestor is in the range of 150 to 250% of the rated RMS voltage, which is considerably lower than the voltage specified for the insulation test. It will be apparent that a motor equipped with an internal arrestor at this voltage rating could not be tested at the higher voltage for the insulation test because the surge arrestor would break down and prevent the higher voltage from being attained. This problem has been an obstacle to the use of permanently connected internal arrestors in sealed motors.
It is a general object of this invention to overcome the foregoing problem by providing an arrestor that has two voltage levels, one for the insulation test and one for normal operation.